1. Field of the Invention
This invention relates to curved laminated transparencies comprising a flexible interlayer having a shade band of uniform tone but a pattern of graded intensity laminated to one or more rigid, transparent sheets of glass or rigid transparent plastic, and to preparing a thermoplastic interlayer for a curved laminated glass assembly having a colored, light-absorbing band in a marginal area of its thermoplastic interlayer. The present invention more especially relates to a method for the preparation of individual interlayers from a continuous ribbon of clear flexible interlayer material and to the use of such interlayers in laminated windshields having more acceptable optical properties and/or improved resistance to impact than those containing interlayers produced by other methods developed in the prior art.
Windshields of automobiles utilize laminated glass assemblies comprising two sheets of glass and a thermoplastic interlayer such as a sheet of plasticized polyvinyl butyral resin or polyurethane. In the current models of automobiles these glass assemblies are bent along the longitudinal axis. In the manufacture of these curved laminated glass assemblies, a thermoplastic interlayer is placed between two matched curved glass sheets and the resulting sandwich is subjected to a laminating process to form a transparent laminated glass assembly. The thermoplastic interlayer is provided with a marginal rectilinear or straight band of a light-absorbing material in order to reduce glare from the sun when the laminated glass assembly is mounted as a windshield in an automobile. The marginal band is preferably graduated in concentration of light-absorbing material with the greatest concentration of light-absorbing dye being present nearest the upper peripheral portion of the thermoplastic interlayer and the dye concentration in the band diminishes gradually toward the lower edge of the band until the amount at the lower edge is almost imperceptible. When such a graduated dyed band in the plastic interlayer is laminated between a pair of curved glass sheets and the resulting laminated windshield is mounted in a tilting or non-vertical fashion, the cut-off point between the dyed band and the undyed portion appears to be curved to a person inside the car. In order to provide an apparent horizontal cut-off line between the dyed band and the non-dyed portion, it is necessary to distort the flat thermoplastic interlayer sheet to curve the boundary between the dyed and the undyed portion in such a manner as to compensate for the curvature that results from differential distortion of the interlayer sheet.
Flat thermoplastic interlayer sheets having a curved colored band suitable for use as interlayers in curved laminated glass assemblies have been prepared from a continuous thermoplastic sheet having a straight colored band by differential stretching of the sheet until the colored band is curved and then heating the sheet while in the stretched condition in order to relieve stresses. The sheets are then cooled to room temperature while still in a stretched condition. Such a method is limited with respect to the amount of stretch that can be used as well as with respect to the degree and type of curvature obtainable.
Thermoplastic sheets stretched differentially by the method described have a non-uniform thickness even when cut from a continuous ribbon of interlayer material of uniform thickness. Furthermore, the heating of the differentially stretched sheet does not eliminate the stresses entirely.
The combination of the residual differential stresses and the difference in thickness resulting from the differential stretching of the thermoplastic sheets (which usually vary about 5 mils (0.13 mm) in interlayer sheets of 30 mils (0.762 mm) nominal thickness) makes it difficult to obtain a uniform bonding between the thermoplastic interlayer sheets and a glass sheet or a pair of glass sheets to produce a laminated safety glass windshield. Laminating assemblies having interlayers of non-uniform thickness in an oil autoclave may result in some oil penetration at the interface between the thinner portion of the interlayer and the glass or other rigid transparent sheet. Furthermore, differential stretching causes certain uneveness in the gradation or texture of the band of shading that is applied to the interlayer material. The art of shaded laminated automobile windshields required some improvement to provide tinted windshields that were not susceptible to delamination and had better optical properties than those available from the prior art.
2. Description of the Prior Art
U.S. Pat. No. 3,455,197 to Richardson and U.S. Pat. No. 3,467,332 to Bachman disclose different types of apparatus for cutting a continuous sheet or ribbon of clear plastic interlayer material into interfitting trapezoidal shapes. Such apparatus are designed for ultimate use of the clear plastic in windshields that do not have dyed portions.
U.S. Pat. No. 3,113,034 to Fix discloses an axially displaceable printing roller whose axial reciprocating movement is correlated with the movement of a continuous ribbon of plastic interlayer material to provide a graded pattern of dye that is repeated along the length of the continuous ribbon. Such ribbon must necessarily be cut with triangular end portions that do not interfit. Furthermore, the axial reciprocation of the printing roller forms rub marks that impair the optical properties of the printed portion of the interlayer.
U.S. Pat. No. 3,593,405 to Beckham discloses a typical prior art technique involving the application of a shade band of graded intensity onto a rectangular sheet of plastic interlayer material. The interlayer material containing the shaded portion of graded intensity is differentially stretched to provide a curved cut-off line so as to appear substantially horizontal when laminated between curved glass sheets and installed in a tilted position in a frame of an automobile.
U.S. Pat. No. 2,933,759 to Startzell discloses an umbrella-type apparatus for simultaneously differentially stretching the opposite side edges of a plurality of layers of a continuous ribbon of plastic interlayer material so that the plurality of layers may be simultaneously differentially stretched when the umbrella-type of plastic stretching apparatus is opened. Stretch marks usually remain in the interlayer sheeting. When the shape requires severe differential distortion, these can extend into the shaded region.
U.S. Pat. No. 3,696,186 to Stark et al discloses apparatus for differentially stretching a continuous ribbon of dyed interlayer material by passing the latter while heated to deformation temperature over a portion of a conical roll and then rapidly chilling the differentially stretched plastic interlayer material. Non-uniform stretching results in non-uniform thickness.
The teachings in the patents enumerated and described previously fail to provide a method of treating clear interlayer material so that a maximum number of interfitting trapezoidal shapes can be provided with a graded shade band along a specific longitudinal edge portion thereof for lamination with curved glass to result in a laminated glass-plastic windshield having a plastic layer of substantially uniform thickness, of substantially uniform tension of a low magnitude merely sufficient to avoid wrinkling and of substantial uniformity of adhesion to the glass throughout the entire extent of the laminated windshield.